1. Field of the Invention
This invention relates generally to the downhole investigation of subterranean formations. More particularly, this invention relates to characterization of a subsurface formation by sampling through perforations in a borehole penetrating the formation.
2. Background Art
Historically, boreholes (also known as wellbores, or simply wells) have been drilled to seek out subsurface formations (also known as downhole reservoirs) containing highly desirable fluids, such as oil, gas or water. A borehole is drilled with a drilling rig that may be located on land or over bodies of water, and the borehole itself extends downhole into the subsurface formations. The borehole may remain ‘open’ after drilling (i.e., not lined with casing), or it may be provided with a casing (otherwise known as a liner) to form a ‘cased’ borehole. A cased borehole is created by inserting a plurality of interconnected tubular steel casing sections (i.e., joints) into an open borehole and pumping cement downhole through the center of the casing. The cement flows out the bottom of the casing and returns towards the surface through a portion of the borehole between the casing and the borehole wall, known as the ‘annulus.’ The cement is thus employed on the outside of the casing to hold the casing in place and to provide a degree of structural integrity and a seal between the formation and the casing.
Various techniques for performing formation evaluation (i.e., interrogating and analyzing the surrounding formation regions for the presence of oil and gas) in open, uncased boreholes have been described, for example, in U.S. Pat. Nos. 4,860,581 and 4,936,139, assigned to the assignee of the present invention. FIGS. 1A and 1B illustrate a known formation testing apparatus according to the teachings of these patents. The apparatus A of FIGS. 1A and 1B is of modular construction, although a unitary tool is also useful. The apparatus A is a downhole tool that can be lowered into the well bore (not shown) by a wire line (not shown) for the purpose of conducting formation evaluation tests. The wire line connections to tool A as well as power supply and communications-related electronics are not illustrated for the purpose of clarity. The power and communication lines that extend throughout the length of the tool are generally shown at 8. These power supply and communication components are known to those skilled in the art and have been in commercial use in the past. This type of control equipment would normally be installed at the uppermost end of the tool adjacent the wire line connection to the tool with electrical lines running through the tool to the various components.
As shown in the embodiment of FIG. 1A, the apparatus A has a hydraulic power module C, a packer module P, and a probe module E. Probe module E is shown with one probe assembly 10 which may be used for permeability tests or fluid sampling. When using the tool to determine anisotropic permeability and the vertical reservoir structure according to known techniques, a multiprobe module F can be added to probe module E, as shown in FIG. 1A. Multiprobe module F has sink probe assembly 14, and horizontal probe assembly 12. Alternately, a dual packer module P is commonly combined with the probe module E for vertical permeability tests.
The hydraulic power module C includes pump 16, reservoir 18, and motor 20 to control the operation of the pump 16. Low oil switch 22 provides a warning to the tool operator that the oil level is low, and, as such, is used in regulating the operation of the pump 16.
The hydraulic fluid line 24 is connected to the discharge of the pump 16 and runs through hydraulic power module C and into adjacent modules for use as a hydraulic power source. In the embodiment shown in FIG. 1A, the hydraulic fluid line 24 extends through the hydraulic power module C into the probe modules E and/or F depending upon which configuration is used. The hydraulic loop is closed by virtue of the hydraulic fluid return line 26, which in FIG. 1A extends from the probe module E back to the hydraulic power module C where it terminates at the reservoir 18.
The pump-out module M, seen in FIG. 1B, can be used to dispose of unwanted samples by virtue of pumping fluid from the flow line 54 into the borehole, or may be used to pump fluids from the borehole into the flow line 54 to inflate the straddle packers 28 and 30. Furthermore, pump-out module M may be used to draw formation fluid from the wellbore via the probe module E or F, or packer module P, and then pump the formation fluid into the sample chamber module S against a buffer fluid therein. This process will be described further below.
The bi-directional piston pump 92, energized by hydraulic fluid from the pump 91, can be aligned to draw from the flow line 54 and dispose of the unwanted sample though flow line 95, or it may be aligned to pump fluid from the borehole (via flow line 95) to flow line 54. The pump-out module can also be configured where flow line 95 connects to the flow line 54 such that fluid may be drawn from the downstream portion of flow line 54 and pumped upstream or vice versa. The pump-out module M has the necessary control devices to regulate the piston pump 92 and align the fluid line 54 with fluid line 95 to accomplish the pump-out procedure. It should be noted here that piston pump 92 can be used to pump samples into the sample chamber module(s) S, including overpressuring such samples as desired, as well as to pump samples out of sample chamber module(s) S using the pump-out module M. The pump-out module M may also be used to accomplish constant pressure or constant rate injection if necessary. With sufficient power, the pump-out module M may be used to inject fluid at high enough rates so as to enable creation of microfractures for stress measurement of the formation.
Alternatively, the straddle packers 28 and 30 shown in FIG. 1A can be inflated and deflated with borehole fluid using the piston pump 92. As can be readily seen, selective actuation of the pump-out module M to activate the piston pump 92, combined with selective operation of the control valve 96 and inflation and deflation of the valves I, can result in selective inflation or deflation of the packers 28 and 30. Packers 28 and 30 are mounted to outer periphery 32 of the apparatus A, and may be constructed of a resilient material compatible with wellbore fluids and temperatures. The packers 28 and 30 have a cavity therein. When the piston pump 92 is operational and the inflation valves I are properly set, fluid from the flow line 54 passes through the inflation/deflation valves I, and through the flow line 38 to the packers 28 and 30.
As also shown in FIG. 1A, the probe module E has a probe assembly 10 that is selectively movable with respect to the apparatus A. Movement of the probe assembly 10 is initiated by operation of a probe actuator 40, which aligns the hydraulic flow lines 24 and 26 with the flow lines 42 and 44. The probe 46 is mounted to a frame 48, which is movable with respect to apparatus A, and the probe 46 is movable with respect to the frame 48. These relative movements are initiated by a controller 40 by directing fluid from the flow lines 24 and 26 selectively into the flow lines 42, 44, with the result being that the frame 48 is initially outwardly displaced into contact with the borehole wall (not shown). The extension of the frame 48 brings the probe 46 adjacent the borehole wall and compresses an elastomeric ring (called a packer) against the borehole wall, thus creating a seal between the borehole and the probe 46. Since one objective is to obtain an accurate reading of pressure in the formation, which pressure is reflected at the probe 46, it is desirable to further insert the probe 46 through the built up mudcake and into contact with the formation. Thus, alignment of the hydraulic flow line 24 with the flow line 44 results in relative displacement of the probe 46 into the formation by relative motion of the probe 46 with respect to the frame 48. The operation of the probes 12 and 14 is similar to that of probe 10, and will not be described separately.
Having inflated the packers 28 and 30 and/or set the probe 10 and/or the probes 12 and 14, the fluid withdrawal testing of the formation can begin. The sample flow line 54 extends from the probe 46 in the probe module E down to the outer periphery 32 at a point between the packers 28 and 30 through the adjacent modules and into the sample modules S. The vertical probe 10 and the sink probe 14 thus allow entry of formation fluids into the sample flow line 54 via one or more of a resistivity measurement cell 56, a pressure measurement device 58, and a pretest mechanism 59, according to the desired configuration. Also, the flow line 64 allows entry of formation fluids into the sample flow line 54. When using the module E, or multiple modules E and F, the isolation valve 62 is mounted downstream of the resistivity sensor 56. In the closed position, the isolation valve 62 limits the internal flow line volume, improving the accuracy of dynamic measurements made by the pressure gauge 58. After initial pressure tests are made, the isolation valve 62 can be opened to allow flow into the other modules via the flow line 54.
When taking initial samples, there is a high prospect that the formation fluid initially obtained is contaminated with mud cake and filtrate. It is desirable to purge such contaminants from the sample flow stream prior to collecting sample(s). Accordingly, the pump-out module M is used to initially purge from the apparatus A specimens of formation fluid taken through the inlet 64 of the straddle packers 28, 30, or vertical probe 10, or sink probe 14 into the flow line 54.
The fluid analysis module D includes an optical fluid analyzer 99, which is particularly suited for the purpose of indicating where the fluid in flow line 54 is acceptable for collecting a high quality sample. The optical fluid analyzer 99 is equipped to discriminate between various oils, gas, and water. U.S. Pat. Nos. 4,994,671; 5,166,747; 5,939,717; and 5,956,132, as well as other known patents, all assigned to Schlumberger, describe the analyzer 99 in detail, and such description will not be repeated herein.
While flushing out the contaminants from apparatus A, formation fluid can continue to flow through the sample flow line 54 which extends through adjacent modules such as the fluid analysis module D, pump-out module M, flow control module N, and any number of sample chamber modules S that may be attached as shown in FIG. 1B. Those skilled in the art will appreciate that by having a sample flow line 54 running the length of the various modules, multiple sample chamber modules S can be stacked without necessarily increasing the overall diameter of the tool. Alternatively, as explained below, a single sample module S may be equipped with a plurality of small diameter sample chambers, for example by locating such chambers side by side and equidistant from the axis of the sample module. The tool can therefore take more samples before having to be pulled to the surface and can be used in smaller bores.
Referring again to FIGS. 1A and 1B, flow control module N includes a flow sensor 66, a flow controller 68, piston 71, reservoirs 72, 73 and 74, and a selectively adjustable restriction device such as a valve 70. A predetermined sample size can be obtained at a specific flow rate by use of the equipment described above.
The sample chamber module S can then be employed to collect a sample of the fluid delivered via flow line 54. If a multi-sample module is used, the sample rate can be regulated by flow control module N, which is beneficial but not necessary for fluid sampling. With reference to upper sample chamber module S in FIG. 1B, a valve 80 is opened and one of the valves 62 or 62A, 62B is opened (whichever is the control valve for the sampling module) and the formation fluid is directed through the sampling module, into the flow line 54, and into the sample collecting cavity 84C in chamber 84 of sample chamber module S, after which valve 80 is closed to isolate the sample, and the control valve of the sampling module is closed to isolate the flow line 54. The chamber 84 has a sample collecting cavity 84C and a pressurization/buffer cavity 84p. The tool can then be moved to a different location and the process repeated. Additional samples taken can be stored in any number of additional sample chamber modules S which may be attached by suitable alignment of valves. For example, there are two sample chambers S illustrated in FIG. 1B. After having filled the upper chamber by operation of shut-off valve 80, the next sample can be stored in the lowermost sample chamber module S by opening shut-off valve 88 connected to sample collection cavity 90C of chamber 90. The chamber 90 has a sample collecting cavity 90C and a pressurization/buffer cavity 90p. It should be noted that each sample chamber module has its own control assembly, shown in FIG. 1B as 100 and 94. Any number of sample chamber modules S, or no sample chamber modules, can be used in particular configurations of the tool depending upon the nature of the test to be conducted. Also, sample module S may be a multi-sample module that houses a plurality of sample chambers, as mentioned above.
It should also be noted that buffer fluid in the form of full-pressure wellbore fluid may be applied to the backsides of the pistons in chambers 84 and 90 to further control the pressure of the formation fluid being delivered to the sample modules S. For this purpose, the valves 81 and 83 are opened, and the piston pump 92 of the pump-out module M must pump the fluid in the flow line 54 to a pressure exceeding wellbore pressure. It has been discovered that this action has the effect of dampening or reducing the pressure pulse or “shock” experienced during drawdown. This low shock sampling method has been used to particular advantage in obtaining fluid samples from unconsolidated formations, plus it allows overpressuring of the sample fluid via piston pump 92.
It is known that various configurations of the apparatus A can be employed depending upon the objective to be accomplished. For basic sampling, the hydraulic power module C can be used in combination with the electric power module L, probe module E and multiple sample chamber modules S. For reservoir pressure determination, the hydraulic power module C can be used with the electric power module L and the probe module E. For uncontaminated sampling at reservoir conditions, the hydraulic power module C can be used with the electric power module L, probe module E in conjunction with fluid analysis module D, pump-out module M and multiple sample chamber modules S. A simulated Drill Stem Test (DST) test can be run by combining the electric power module L with the packer module P and the sample chamber modules S. Other configurations are also possible and the makeup of such configurations also depends upon the objectives to be accomplished with the tool. The tool can be of unitary construction a well as modular, however, the modular construction allows greater flexibility and lower cost to users not requiring all attributes.
The individual modules of the apparatus A are constructed so that they quickly connect to each other. Flush connections between the modules may be used in lieu of male/female connections to avoid points where contaminants, common in a wellsite environment, may be trapped
Flow control during sample collection allows different flow rates to be used. In low permeability situations, flow control is very helpful to prevent drawing formation fluid sample pressure below its bubble point or asphaltene precipitation point.
Thus, once the tool engages the wellbore wall, fluid communication is established between the formation and the downhole tool. Various testing and sampling operations may then be performed. Typically, a pretest is performed by drawing fluid into the flow line by selectively activating a pretest piston. The pretest piston is retracted so the fluid flows into a portion of the flow line of the downhole tool. The cycling of the piston through a drawdown and buildup phase provides a pressure trace that is analyzed to evaluate the downhole formation pressure, to determine if the packer has sealed properly, and to determine if the fluid flow is adequate to obtain a diagnostic sample.
It follows from the above discussion that the measurement of pressure and the collection of fluid samples from formations penetrated by open boreholes is well known in the relevant art. Once casing has been installed in the borehole, however, the ability to perform such tests is limited. There are hundreds of cased wells which are considered for abandonment each year in North America, which add to the thousands of wells that are already idle. These abandoned wells have been determined to no longer produce oil and gas in necessary quantities to be economically profitable. However, the majority of these wells were drilled in the late 1960's and 1970's and logged using techniques that are primitive by today's standards. Thus, recent research has uncovered evidence that many of these abandoned wells contain large amounts of recoverable natural gas and oil (perhaps as much as 100 to 200 trillion cubic feet) that have been missed by conventional production techniques. Because the majority of the field development costs such as drilling, casing and cementing have already been incurred for these wells, the exploitation of these wells to produce oil and natural gas resources could prove to be an inexpensive venture that would increase production of hydrocarbons and gas. It is, therefore, desirable to perform additional tests on such cased boreholes.
In order to perform various tests on a cased borehole to determine whether the well is a good candidate for production, it is often necessary to perforate the casing to investigate the formation surrounding the borehole. One such commercially-used perforation technique employs a tool which can be lowered on a wireline to a cased section of a borehole, the tool including a shaped explosive charge for perforating the casing, and testing and sampling devices for measuring hydraulic parameters of the environment behind the casing and/or for taking samples of fluids from said environment.
Various techniques have been developed to create perforations in cased boreholes, such as the techniques and perforating tools that are described, for example, in U.S. Pat. Nos. 5,195,588; 5,692,565; 5,746,279; 5,779,085; 5,687,806; and 6,119,782, all of which are assigned to the assignee of the present invention.
The '588 patent by Dave describes a downhole formation testing tool which can reseal a hole or perforation in a cased borehole wall. The '565 patent by MacDougall et al. describes a downhole tool with a single bit on a flexible shaft for drilling, sampling through, and subsequently sealing multiple holes of a cased borehole. The '279 patent by Havlinek et al. describes an apparatus and method for overcoming bit-life limitations by carrying multiple bits, each of which are employed to drill only one hole. The '806 patent by Salwasser et al. describes a technique for increasing the weight-on-bit delivered by the bit on the flexible shaft by using a hydraulic piston.
Another perforating technique is described in U.S. Pat. No. 6,167,968 assigned to Penetrators Canada. The '968 patent discloses a rather complex perforating system involving the use of a milling bit for drilling steel casing and a rock bit on a flexible shaft for drilling formation and cement.
Despite such advances in formation evaluation and perforating systems, a need exists for a downhole tool that is capable of perforating the sidewall of a wellbore and performing the desired formation evaluation processes. Such a system is also preferably provided with a probe/packer system capable of supporting the perforating tool and/or pumping capabilities for drawing fluid into the downhole tool. It is further desirable that this combined perforating and formation evaluation system be provided with a bit system capable of even long term use, and be adaptable to perform in a variety of wellbore conditions, such as cased or open hole wellbores. It is further desirable that such as system provide a probe/packer assembly that is less prone to the problems of differential sticking of the tool body to the borehole wall, and reduces the risk of damaging the probe assembly during conveyance. It is further desirable that such a system have the ability to perforate a selective distance into the formation, sufficient to reach beyond the zone immediately around the borehole which may have had its permeability altered, reduced or damaged due to the effects of drilling the borehole, including pumping and invasion of drilling fluids.